Current use of thermal solar plants

Plastic absorbers to heat water for swimming pools

Due to their limited pressure and temperature durability, plastic absorbers are mainly used for heating pool water. In this case, the desired temperature level is only a few degrees higher than the ambient temperature. Thus, simple plastic absorbers, which can usually be mounted uncovered on a flat roof due to their low operating temperature, are sufficient. Since they consist entirely of plastic, they have the advantage of single-circuit operation. The chlorinated pool water is directly pumped through the absorbers by a circulation pump and no heat exchanger is needed.

If a filter pump already exists, it can also be used for the solar circuit. In this case, the adequate dimensioning of the pump is very important. Plastic collectors are only operated during the summer months and have to be emptied before the first frost sets in.

Figure 14a: Solar plant for pool heating

Thermal solar systems for preparation domestic hot water (single family houses)

In Central Europe, domestic hot water is usually heated either by using electricity or by a boiler operated with solid fuels, gas or oil. If this heating system is used in summer only to heat water, the boiler operates with an extremely low efficiency. During this period, the domestic hot water can be heated in an ecologically beneficial and economic way by a solar heating system.

During the summer the energy supplied by the sun is sufficient to cover between 80% to 95% of the hot water demand, depending on the dimensioning of the system. During the transitional period and winter months, the solar energy supply is still sufficient to pre-heat the domestic water, i.e. the temperature of the fresh water has to be raised only by a small amount by the heating boiler or electric heating element. In the cold winter months, water temperatures between 30 to 50°C can still be reached on sunny days. Thus, the energy saving effect in winter may still be considerable. The past few years have shown that these thermal solar systems are already technically mature and reliable.

Description for a thermal solar system to prepare domestic hot water

Incoming solar radiation is converted by the collector into heat. This heat is transported by a heat transfer medium (water/anti-freeze mixture) in pipes to a storage tank. There the heat is transferred through a heat exchanger to the domestic water. The storage tank should be dimensioned in such a way that its volume corresponds to the hot water demand of at least two days. The installation of an additional (e.g. electric) heater ensures that sufficient amounts of domestic hot water are available even during long and continuous periods of overcast weather.

Concerning the hydraulic scheme (see figure 15) the heat transfer medium is circulated by a circulation pump (1). An electronic control system ensures that the pump is only turned on when an energy gain from the solar collector is expected, i.e. when the medium in the collector is warmer than the domestic hot water in the tank. Both the storage tank and the pipes are well insulated to avoid unnecessary losses.

Additionally, thermometers (4, 7) in the loading preliminary and reverse pipe belong to the basic equipment of the system. They are installed preferably close to the storage tank. Temperature-dependent volume changes in the fluid are compensated by the expansion tank (8), keeping the operating pressure in the system constant.

The gravity brake (2) prevents the heat from flowing back to the top if a standstill occurs in the system. A pressure relief valve (5) allows the fluid to escape if the system pressure becomes too high. An air escape valve (6) is installed at the highest point allowing air in the pipes to escape. Inlet and outlet taps complete the system (10).

Figure 15. Illustration of a hydraulic scheme

Figure 15a. Single family houses with solar hot water systems

Solar plants in multi family houses

Approximately 43% of the Austrian population lives in multi family houses(ÖSTAT, 1991). As a rule the hot water is prepared in these houses using decentralised systems. A common supply system for a dwelling comprises a small storage tank (100 to 200 l) heated using electricity (night power) to cover consumption in the bath room as well as an electrically heated flow heater (5 to 10 l) to cover smaller consumption rates in the kitchen.

By the end of 1999, approximately 500 thermal solar plants were installed in Austria in multi family houses. If one considers this fact then one clearly recognises market potential for solar plants in this field. Apart from the existing market potential, multi family houses favour the installation of solar plants since they are of a compact design compared to single family houses. Whereas naturally only decentralised small-scale plants are installed in single family houses, larger, central solar plants could be realised in such multi family houses. This leads in greater potential to save on CO2 with less system costs.

However we are far away from accomplishing the wide-spread market introduction of solar plants in Austria. Co-operative residential building associations tend to be sceptical of solar technologies since they require additional planning, co-ordination and financial input when it comes to constructing or reconstructing of the building. In contrast to private "home builders", where the decision to install a solar plant is mainly of an emotional nature, economic considerations tend to dominate amongst co-operative residential building associations.

Figure 16: Hydraulic scheme of a thermal solar system for a multi family house (energy storage tank and stand-by storage tank)

Figure 16a. Hot water preparation in a multi family house

Combisystems for solar space heating

At the current moment in time fossil forms of energy (oil, gas and coal) mainly provide space heating. In a similar way to the wide-spread market introduction of solar plants for preparation of domestic hot water it is once again the private builder who has realised the very first solar space heating in the past The characteristics of the plant and the yields which can be obtained with regard to numerous plants were recorded and evaluated. The results led to considerable system optimisations. In this field Austria has assumed a pioneering role by European comparison. In 1998 approximately 50% of the complete collector area installed in buildings was accounted for by solar space heating. This corresponds to around 20 to 25% of the plants totally installed /1/.

As the energy supply is inversely proportional to the energy demand - i.e. during summer when only little energy for heating purposes is required, the energy supply is high, and during winter when much energy is needed, the supply is low - the key question is how to store the energy from the summer to the winter.

Various systems completed in recent years demonstrate that it is possible to store the heat from summer to the winter in large water tanks (seasonal storage with 50 to 80 m3 for 100 m² living area), and thus to use only solar energy for heating. From an economical point of view, seasonal storage for single family houses and two-family houses is quite expensive and so not generally applicable.

The second economically more interesting concept for single family houses is that of partly solar space heating. If collector areas of 20 to 50 m² are combined with storage tanks (1 to 5 m3) which are able to store heat for some hours (overnight) or for some days or for some weeks, solar fractions of up to about 20 to 60% can be achieved at reasonable costs compared to systems with seasonal storage. The remaining energy consumption is ideally covered by a wood combustion plant (piece wood boiler, pellets or wood chip boiler).

Figure 17: Hydraulic scheme of a partly solar space heating system (central energy storage tank and external heat exchanger for preparing domestic hot water

Figure 17a: Single family house with solar space heating

Solar Assisted Biomass Local Heating Network

These plants for the supply of heat (space heating, preparation of water for domestic use) principally comprise biomass combustion, the solar plant including the storage tank and the local heating network. As a result of the combined use of energy from biomass and solar radiation, heat can be comfortably supplied to villages and towns from renewable sources of energy. These combined plants should in the main be used in applications where for economic and ecological reasons it does not make sense to heat with biomass combustion in the summer months. Since according to experience lots of customers only join the local heating network when services are offered all year round it makes sense to prepare warm water in the summer months using a central solar plant. This is normally assembled directly on the roof of the building for the biomass heating plant central (boiler room and biomass fuel storage hall). For the reasons mentioned several operators of biomass local heating networks have added a thermal solar system to their plants in the past.

It is in this respect extremely positive that it is not just small local heating networks which use solar energy but rather there is a trend towards biomass heating plants for large collector areas (collector area larger than 1.000 m²). It has been seen that the use of large area collectors (8 to 12 m²) has advantages at the assembly stage. Apart from the collector area, the correct dimensioning of the storage tank is also very important. In these storage tanks the solar heated medium is disposed in layers in a temperature-oriented manner and transported to the consumers via the local heating network. In the three months in the summer it is possible to obtain a solar fraction of more than 90% and subsequent heating is only necessary in longer periods of bad weather. So a backup system with a small biomass or oil boiler should be used. With the storage tank installed for the solar plant it is possible to accommodate peak capacity in the short-term in the winter heating period so that the biomass boiler can be dimensioned smaller which once again has a very positive effect on the investment costs.

Recent plants

So far there are 15 solar biomass heating networks with a collector area of up to 1,250 m² in operation. The following table shows all the solar biomass heating plants operating in Austria.

The first solar biomass heating network in Austria (Deutsch Tschantschendorf) built in 1994 was monitored from 1995 to 1997 by the operator and AEE.

Figure 18a: Heat from the sun and from biomass

Solar air heating systems

Concerning air collectors one means sun collectors which use air as a heat transfer medium. Starting with the first developments at the end of the 19th century in the USA, a number of different solar air technologies have been developed around the world without these however being widely spread so far.

Air heating systems are suitable for applications which require direct warm air. Good applications for air collectors are indicated in the examples which follow:

• Systems for space heating: In air heating systems which are used in the building sector one has to differentiate between the direct and indirect introduction of air. The direct introduction of air is given preference when heating halls and storage rooms. In singe and multi family houses, office buildings etc. the air is introduced mostly indirectly or by a combination of direct and indirect. The indirect introduction of air is performed by hyposcausts respectively murocaust systems or sometimes also using intermediate wall systems. In this respect the heat release in the rooms is performed in the form of heat radiation. The direct introduction of air takes place via controlled aeration and ventilation. In general rock storage tanks or storage walls are used for heat storage in air heating systems. Air heating systems can accommodate a large amount of the space heating requirements provided that the overall system is correctly dimensioned. Complete coverage is, however, not possible due to unfavourable solar energy conditions in the winter.
     
• Plants for drying agricultural or commercial products such as cereals, seeds, medicinal plants and herbs, building materials, wood etc. The drying potential of the air collector plants equals about 0.2 to 0.7 kg of water per hour and m² of collector area. The drying of sewage sludge is another interesting application.

Figure 19: Hydraulic diagram of an air heating system with a hypocaust for heat release and a domestic water storage tank for the preparation of domestic water in the summer

Figure 19a: Solar air system in a single family house

Solar cooling

In Central-European regions air conditioning in the summer is as a rule only employed in large administrative buildings, in commercial buildings etc. whereas in Southern Europe air conditioning systems are being also used increasingly in the residential sector. Since the time of the year in which air conditioning corresponds very well with the time of the year for solar radiation, it makes sense to develop solar supporting systems for air conditioning in the summer.

Apart from thermal processes which involve the sorption technique, the photovoltaic technique with compression cooling plants currently plays a role when it comes to the sector for the generation of coldness using solar energy /6/.

Processes are favourable for the use of thermal solar collectors in which the temperatures required to generate coldness are as low as possible so as to be able to operate the collector field efficiently. Roughly these processes divide on the one hand into the type of process control - open or closed - and on the other hand into the type of sorption agents used - solid or liquid.

Apart from the preparation of domestic hot water and partly solar space heating, solar air conditioning is another important application for the use of thermal solar energy in the building sector. Up until now plants have mostly been constructed within the framework of subsidised demonstration projects. An IEA-Task was started to promote research activities within the field of solar air conditioning.

Transparent thermal insulation (TWD)

The principle of transparent thermal insulation is that when using transparent thermal insulation materials (TWDM) heat losses are at least partly compensated by solar gains respectively it can be used as heat gains (e.g. for the heating of buildings) compared to conventional thermal insulation in which as a rule only transmission losses are reduced. Thus compared to light impenetrable (opaque) insulation materials, TWDM's have two main properties which are of energetic importance:

• a good heat-insulating effect (i.e. the lowest possible values for the thermal diffusion coefficient k-value)
• a high permeability for solar radiation (i.e. the highest possible values for the overall energy diffusion rate g)

TWD materials obtain their thermal insulation effect as a result of a high content of air and by subdividing the air layers into small volumes. As far as the geometric structure and the materials are concerned, in principle absorber-parallel layer structures, absorber-vertical structures (channel, honeycomb and capillary structures), chamber structures (grouser plates, transparent foam, hollow balls) and quasi-homogeneous materials (aero-gels) made of glass or transparent plastics are possible. The TWD structures most frequently used are absorber-vertical tubes or honeycomb structures manufactured in the extrusion processes of light-permeable plastics such as polymethylmetacrylat (PMMA) and polycarbonate (PC).

Depending on the layer thickness of the TWD structure, values of k = 0,7 - 1,5 W/(m²K) are typically reached for the heat losses when used behind a glass plate to protect against climate effects and gdiff = 50 to 70% for the diffuse-hemispheric overall energy diffusion rate.

The systematic development of so-called transparent thermal insulating materials commenced really only at the beginning of the 1980's and has led to a range of new concepts for thermal solar energy utilisation.

In this respect the most important application is the transparent insulation of the outer walls of buildings. Here solar energy is directly attained without any supporting energy and without the help of moved parts. In these passive systems even a very small amount of radiation causes a reduction in the heat losses, with a medium amount of radiation losses and gains balance one another and when there is more sun this will result in net heat gains.

Another application which is already very widely spread is the use of TWD elements in the window sector where no look-through is required. Systems of this kind are deliberately employed with skylights to control and distribute the light.

New developments deal with the use of TWD's in special collectors to prepare warm and hot water. If a TWD is attached to the side and towards the back of a very well insulated flat collector, then similar to a vacuum-tube collector, very high operating temperatures can be reached (100 - 150°C for the preparation of process heat) with a good rate of efficiency. In the event of a standstill - absorber temperatures of 260°C were measured - the TWD materials are exposed to a high temperature load which means that glass capillaries are necessary.

Whilst until now only isolated projects have been realised with TWD-systems in Austria, there is a range of test houses with TWD-facades in other European countries (BRD, CH, UK). The results to date lead us to expect that the use of TWD systems in buildings, both new and old, will make a considerable contribution towards the saving of fossil sources of fuel /3/.

Figure 21: Terrace house estate with TWD - facade

Figure 21a: Example for a TWD-material

[ previous chapter 4 ] <= [ Technology Profile ] => [ References ]

Sources

/1/ G. Faninger, Solarmarkt in Österreich, Bundesverband Solar, 1998

/3/ A. Indinger, R. W. Lang, H. Wilk, W. Weiß, Endbericht: Österreichisches Netzwerk für Nachhaltige Wirtschafts- und Technologieentwicklung, Aktionsschwerpunkt Solarenergie, 1999

/6/ F.N. Fett, Photovoltaische und thermische solare Kühlung im Vergleich, Projekt-Zwischenbericht